There have been developed and proposed various metal pipes which are coated with a thermoplastic synthetic resin such as polyethylene, polyvinylchloride and the like. For example, it is known to produce metal pipes coated with polyethylene by depending powder of polyethylene on the outer surface of a preheated metal pipe and melting the polyethylene powder completely by a heat treatment. The coated metal pipes of this sort show good initial adhesion between the polyethylene coat layer and the metal pipe but the adhesion between the two parts is soon lowered during use of the coated pipes due to the inferior water resisting property of the coated layer.
It is also known to use an adhesive when coating metal pipes to adhere the coating material to the metal pipe surfaces with the aid of the adhesive. For instance, ethylene copolymers such as ethylene-vinyl acetate copolymer, and ethylene-ethylacrylate copolymer are known to be useful as adhesives in the production of coated metal pipes. The coat layers of the resulting coated pipes show relatively good initial adhesion but, once damaged, they are easily peeled off over a wide area due to poor water resistant property of the adhesive.
Also known in the art is the method of coating metal pipes with use of a hot-melt type adhesive tape which has a plasticizer and a tackifier blended with a basic rubber material, as described in U.S. Pat. Nos. 3,802,908 and 3,823,045. Such hot melt type adhesive hardens and becomes brittle under low temperature conditions as in winter, loosing its ability of adhesion to a considerable degree to lower the adhesion between the adhesive layer and the coat layer as well as the shock resistance thereof. On the other hand, the hot metal type adhesive softens when exposed to high temperatures in summer to result in a lowered adhesive property and tends to exude when a load is imposed on the pipe, as a result lowering the adhesion as well as the resistance to corrosion.